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Boiler Fault

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I’m hoping someone can help with this boiler problem. Couple have just moved into a house that has a Grant Vortex combi boiler. The current setup is a two channel programmer, turn the heating on at programmer & it calls for heat at stat that then returns to CH terminal inside boiler. Hot water from programmer on is wired to DHW port in boiler. Now it all works fine heating wise when it’s only the CH that’s on at the programmer but when the DHW is on along with the CH even when the heating stat is satisfied at say 15, a while later it fires the heating back up again & is absolutely roasting & won’t turn off. It’s way more than 15 deg I’m told. I’ve only looked at it briefly, tested to prove that the programmer & stat were doing as they should ie switching which they were. I’m due to go back a look to investigate further so any tips appreciated. I’ve noticed there’s still a link between terminals 25 & 26 I think it was. The room stat terminals. Should this be removed even though the stat is not wired from these terminals, it’s wired from the programmer?
 
This doesn't quite add up.
You say "combi". That would normally heat hot water on demand based on a pressure switch in the boiler detecting when a tap is turned on, and not require a programmer telling the boiler to heat water.

Is there a hot water cylinder or not? If so are there 2 x 2 port zone valves or a single 3 port zone valve?
Can you get the exact model of the boiler?
 
Pretty sure a Grant Vortex is an oil fired combi and these do require a HW programmer to prevent them firing up 24/7 as they cycle to maintain their fairly large volume of internally stored HW. Does sound like the link you mention might be overriding the roomstat, is this a newly installed system?
 
This doesn't quite add up.
You say "combi". That would normally heat hot water on demand based on a pressure switch in the boiler detecting when a tap is turned on, and not require a programmer telling the boiler to heat water.

Is there a hot water cylinder or not? If so are there 2 x 2 port zone valves or a single 3 port zone valve?
Can you get the exact model of the boiler?

Grant Vortex Combi 26e.
Pretty sure a Grant Vortex is an oil fired combi and these do require a HW programmer to prevent them firing up 24/7 as they cycle to maintain their fairly large volume of internally stored HW. Does sound like the link you mention might be overriding the roomstat, is this a newly installed system?

Aye this sounds like the setup. It’s been in for years, new tenants just moved in so no idea if it’s always been like this. Link been removed today & still no different, all that does is cut the CH pump out.
 
Linking out the stat terminals sounds right if the room stat is before the boiler.
The combis do have a water jacket that is kept up to temp so can fire regularly if it's an external boiler.
I'm trying to fish through my unreliable memory and all I can come up with is a frost stat in a cold boiler house and sticky valves.
 
Sorry my comment was unhelpful.
I've had a gander at the installation manual for a 26e.
At it's simplest, the DHW terminal can be wired permanently live to just do water on demand.
The manual suggests a room thermostat in series with the heating channel of the programmer.

Another thought is whether anything is connected to terminals 23 and 24, which would be for an additional frost stat.
(It says there is an integral frost stat in the boiler so this would be for another one)

It honestly sounds as though you should start again from first principles checking one thing at a time, as you were originally.

if you haven't found it btw here's the manual.

 
Meant to give an update Friday night but forgot. Have proven it’s not the programmer or stat. It’s firing 240v on the CH pump terminal out of nowhere, even when there is no call for heat & 240v at the CH terminal. Could it be PCB problem? Like I said, it’s not happening at all when the DHW is off.
 
I have just looked at the wiring diagram on line and there are a lot of internal links which if in the wrong terminal could put 230 volt onto pump, not sure what actual model you have but terminals 7 to 12 which is a stat link could if linked in will over ride the stat and permanently put a supply to the pump and bypass the stat, just a thought
 
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